1. Field of the Invention
The present invention relates generally to power supply apparatus and more particularly to an improved DC to DC converter of the type used to provide power for computers and other electronic devices.
2. Description of the Prior Art
DC to DC converters obtain power from an input source and convert it into regulated output power for delivery to a load. However, not all of the input power is converted to output power. Some is dissipated as heat within the converter. The converter's efficiency is defined as the ratio of delivered output power to converter input power. Heat is normally removed from such converters through the use of metal base plates or heat sink devices thermally coupled to but electrically isolated from all internal heat generating components. The basic thermal design problem is to transfer heat from the heat generating components to the base plate and then from the base plate into the surrounding environment.
It is well understood that heat is transferred from regions of high temperature to regions of low temperature via radiation, conduction and convection. While all three mechanisms are active to some degree in every converter, convection will normally be the dominant heat transfer mechanism in most applications.
Although radiant heat transfer occurs continuously between objects at different temperatures which are exposed to each other, the net effect on the temperature of an individual part is dependent on a great many factors including its temperature relative to the other parts, the orientations of the various parts and their surface finishes and spacing. In most cases radiation will account for only a small percentage of the total heat transfer in a converter.
Heat is normally conducted to the base plate of a converter through mechanical connection and surrounding potting materials and then into an attached heat sink or heat conducting member. Heat conducted to the base plate and across the interface between the base plate and a mating member will result in a temperature drop which must normally be controlled. In some cases the heat can be conducted from the base plate of the converter to a remote dissipative surface via a thermally conductive member. The resulting base plate temperature will be the sum of the temperature of the dissipative surface to which the base plate is attached, the temperature rise in the heat conducting member and the rises across the two surface interfaces. Minimizing total temperature rise is dependent on controlling interface resistance and controlling the thermal resistance of the transfer member through appropriate material selection and dimensioning.
Convective heat transfer into air is by far the most commonly used method for cooling modern converters. Free or natural convection refers to a heat transfer from a dissipative surface into a cooler surrounding mass of otherwise still air. Forced convection refers to heat transfer into a moving air stream. Converter temperature depends on the temperature of the air, total dissipated power and the values of two thermal resistances: the thermal resistance of the surface interface between the base plate and the heat sink, and the heat sink to air thermal resistance. Through use of thermally conductive potting materials, heat transfer both to air from the external surfaces of the potted device and to the base plate can be improved
Whereas the surface interface resistance can be minimized by the conduction characteristics of the device, the heat sink to air resistance is dependent on a variety of factors including heat sink material and geometry, air temperature, air density and air flow. As designers have attempted to reduce the size and increase the power density of the DC to DC converter, they have attempted to combine the component carrying circuit boards with the base plate to form a metal-backed PC board which can be mounted directly to a heat sink. While this provides a degree of improvement and allows substantial reduction in the size of the device, it suffers the disadvantage that it subjects the control electronics to elevated temperatures.